1. Field of the Invention
The present invention relates to x-ray detector systems, and in particular to x-ray detector systems having an array of detector elements with a collimator disposed in front of the detector elements in the direction of radiation propagation, with one detector element being associated with each channel in the collimator.
2. Description of the Prior Art
Radiation detector systems are required in most types of x-ray examination devices, particularly devices used in radiographic medical diagnosis wherein an x-ray penetrates a subject under examination and the intensity distribution of the radiation is analyzed. X-ray detector systems used, for example, in computer tomography and computer radiography may have between several hundred and several thousand individual detector elements which are linearly or circularly arranged and directed toward the source of x-rays, such as an x-ray tube. In many installation, a collimator is disposed in the direction of radiation propagation in front of the detector array. The collimator has a plurality of spaced plates consisting of highly radiation absorbent material directed toward the x-ray source for suppressing scattered radiation generated in the measuring field. All x-ray quanta which arrive through the measuring field along a straightline path from the x-ray source should be documented or recorded to as great as extent as possible by the detector system. In order to achieve the best results, as few as possible of the quanta should be lost due to the geometrical structure of the array of detector elements and the collimator. Recording of as many x-ray quanta as is possible is of significant advantage because, given a fixed x-ray power, the number of documented x-ray quanta defines the image quality, among other factors. Although it would be possible, for example, to increase the number of recorded quanta by increasing the radiation dose to the patient, this is undesirable because of possible danger to the patient as well as because of increasing the x-ray tube load. Increasing the x-ray tube load increases the number of movable image artifacts as does lengthening of the exposure times.
A detector system is described in German OS No. 28 40 965 corresponding to U.S. Pat. No. 4,292,525. In this system, the detector array is arranged in an arc around the tube focus, and the detector elements are partially disposed within the collimator channels associated therewith. The wall of the collimator channels shield the detector elements against lateral scattered radiation which may potentially occur.
As particularly seen in FIG. 4 of the U.S. Pat. No. 4,292,525, the radiation permeating a collimator channel is only partially incident on the effective detector surface, i.e., the surface of the scintillation crystal. A not inconsiderable portion of the x-radiation proceeds past the scintillation crystal and is thus no longer available for documentation. The encapsulation of the scintillation crystal forms dead zones in which x-radiation can no longer be detected. A value known as the filling factor is a measure for the x-ray quanta actually incident on the radiation detector surface relative to the total number of x-ray quanta arriving at the detector element. In order to achieve a higher filling factor, it is therefore preferable to avoid the lateral dead zones, i.e., lateral detector regions which no longer register incident radiation, and to increase the effective detector area by enlarging the radiation transducer on which the radiation is incident. A maximum percentage of the x-ray quanta passing through the collimator channels in parallel should thereby be documented.